On the top of; upon; on. See On to, under On, prep. [1913 Webster]

onto, prep. disp. to a position or state on or in contact with (cf. on to).

Usage:
The form onto is still not fully accepted in the way that into is, although it is in wide use. It is however useful in distinguishing sense as between we drove on to the beach (i.e. in that direction) and we drove onto the beach (i.e. in contact with it).

get onto, leech onto, onto land